An electric power converting device converts DC power from a DC power supply circuit into AC power and supplies it to a motor generator. PTL 1 discloses an apparatus which detects the ground fault between a DC power supply circuit or an AC circuit, and a vehicle body. According to this technique, an AC signal generation circuit that outputs an AC signal with a constant output impedance is connected to a DC power supply circuit and an AC circuit via a capacitor. When the ground fault occurs in the DC power supply circuit or the AC circuit, the output impedance of the AC component in the AC signal generation circuit will vary, and a voltage level of the AC signal will vary. The occurrence of the ground fault is detected by detecting a change in the voltage level of the AC signal by the voltage level detection circuit.